DE3779575T2 - METHOD FOR CONTROLLING A DISPLAY DEVICE AND A DISPLAY DEVICE SUITABLE FOR SUCH A METHOD. - Google Patents

Description

The invention relates to a method for controlling a display device with an electro-optical display medium between two support plates, a system of picture elements arranged in rows and columns, each picture element being formed by picture electrodes which are attached to mutually facing surfaces of the support plates, and a system of row electrodes and column electrodes, a row of picture elements being selected via the row electrodes with the aid of non-linear switching elements connected in series with the picture elements, and data signals being offered via the column electrodes.

The invention also relates to a playback device in which such a method can be used.

In this context, it should be noted that, if desired, the terms row electrode and column electrode are interchangeable in this application, so that when reference is made to a column and a row electrode, a row electrode or a column electrode may also be meant.

A display device of this type is suitable for reproducing alphanumeric and video information using passive electro-optical display media such as liquid crystals, electrophoretic suspensions and electrochromic substances.

A display device of the type mentioned at the beginning, in which "back-to-back" diodes are used as switching elements, is known from the American patent US-A-4,223,308. By using switching elements, a storage function is obtained so that the information offered to a controlled line remains sufficiently present in a picture element during the time in which the other row electrodes are controlled. However, due to capacitive crosstalk as a result of the capacitance of the non-linear switching elements, this information can have a varying value, since the same columns are used to offer data signals when selecting different rows of picture elements.

The voltage on a picture element can then change in such a way change so that the transmission level (gray level) becomes higher or lower than the desired value. If the gray levels are to be determined exclusively via the transmission curve, the number of gray levels, in relation to the maximum signal level, is greatly limited by the crosstalk mentioned.

The crosstalk resulting from signal changes depends primarily on the capacitance of the nonlinear switching elements.

Another possibility for realizing grayscale is to divide a picture element into a number of sub-segments in which the proportion of the number selected sub-segments determines the grayscale. This requires additional control with additional column electrodes.

Such a division without additional control can also be used to provide a certain redundancy, since connections can fail. This division generally results in smaller sub-elements, for which smaller image electrodes are used. However, this leads to the capacitance of the image elements decreasing (relatively) with respect to that of the non-linear switching elements. This increases crosstalk.

The object of the present invention is to provide a method of the type mentioned at the outset in which the disadvantages mentioned are largely eliminated.

To achieve this object, a method according to the invention is characterized in that a column electrode is provided with a data signal during part of the time period available for the selection of a picture element row, this data signal being provided almost simultaneously with a selection signal of the row electrode belonging to the picture element row, and in that a non-selection signal is provided to the row electrode during the other part of the time period available for the selection and in that in the absence of a data signal a reference voltage is applied to the column electrode.

In television applications, the reference voltage is preferably determined by the average value of the minimum data signal voltage in a first field and the maximum data signal voltage in a second field.

Preferably, a value of 0 volts is chosen for the reference voltage mentioned.

In the non-prepublished Dutch patent application 86018041, equivalent to EP-A-0253243 (PHN 11.811), the applicant proposes a method in which a data signal, after selection of a line and before selection of a following line, changes its sign with respect to a reference voltage determined by the mean value of the minimum data signal voltage in a first (odd) field and the maximum data signal voltage in a second (even) field, and in which the energy content of the partial signal having a positive sign with respect to the reference voltage is substantially identical to that of the partial signal having a negative sign with respect to the reference voltage.

This compensates for the crosstalk by generating a crosstalk signal with an opposite sign and almost identical energy content.

In an embodiment described in the above-mentioned application, the data signal consists of two partial signals with almost identical absolute voltage values and a duration of practically half the line duration. The signals with opposite signs can be obtained using simple inverter circuits.

Switching can be very fast, especially when using fast non-linear switching elements such as diode rings.

The present invention is based on the realization that when using the fast switching elements, the crosstalk can be reduced even further if the data signal is offered for a period of time that is short in relation to the maximum time available for selection. The shorter the time in which the data signal is offered, the more the crosstalk decreases; it can decrease to such an extent that it is no longer necessary to divide the data signal into sub-signals with opposite signs.

Nevertheless, the advantages of such a division remain, of course. A particular method according to the invention is characterized in that before the reference voltage is applied to the column electrode, the data signal changes its sign with respect to the reference voltage and the energy content of the partial signal thus obtained, which has a positive sign with respect to the reference voltage, is almost identical to that of the partial signal which has a negative sign with respect to the reference voltage, while one of the partial signals almost coincides with the selection signal.

The fast switching times make the process suitable for colour television applications with double the number of lines (HDTV = high-definition television) is interesting.

Since the crosstalk mentioned has now become practically negligible, the picture elements can be divided into a large number of sub-elements in order to obtain redundancy. A device for applying a method according to the invention with an electro-optical reproduction medium between two carrier plates, a system of picture elements arranged in rows and columns, each picture element being formed by picture electrodes which are attached to mutually facing surfaces of the carrier plates, and a system of row electrodes and column electrodes for controlling the picture electrodes with the aid of non-linear switching elements, is characterized in that a column electrode is connected to a connection for a signal to be reproduced or to a connection for a reference voltage via a parallel connection of two branches with complementary operating switches.

A display device of the type described can also be characterized in that a picture electrode is divided into a number of partial electrodes, each of which is operated via at least one non-linear switching element.

In a playback device in which crosstalk compensation is used, the branch for the signal to be reproduced contains two sub-branches with switches, one of the sub-branches containing an inverter circuit which is connected in series with the switch.

Complementary switches mean that one switch is opened when the other is closed and vice versa.

The playback device preferably also contains a driver circuit for the (complementary) switches.

Embodiments of the invention are shown in the drawing and are described in more detail below. They show:

Figure 1 is a schematic representation of a section through part of a display device in which the invention is applied,

Figure 2 shows schematically a transmission/voltage characteristic of a reproduction cell in such a reproduction device,

Figure 3 shows schematically part of a driver circuit for such a playback device,

Figure 4 shows a schematic equivalent circuit diagram of an element of such a playback device,

Figure 5 is a schematic plan view of a display cell,

Figure 6 shows a modification of the display cell of Figure 5,

Figures 7 and 8 show schematically in the circuit of Figure 3 the signals occurring when applying a method according to the invention,

Figure 9 shows a schematic circuit for realizing such signals.

Figure 1 is a schematic representation of a section through a part of a display device 1 with two carrier plates 2 and 3, between which a liquid crystal 4 is located. The inner surfaces of the carrier plates 2 and 3 are provided with electrically and chemically insulating layers 5. A large number of image electrodes 6 and 7 arranged in rows and columns are attached to the carrier plates 2 and 3, respectively. The image electrodes 6 and 7 facing each other form the image elements of the display device. Strip-shaped column electrodes 11 are attached between the columns of image electrodes 7. The column electrodes 11 and the image electrodes 7 can advantageously be integrated to form strip-shaped electrodes. Strip-shaped row electrodes 8 are attached between the rows of image electrodes 6. Each image electrode 6 is connected, for example, to a row electrode 8 using a diode 9 (not shown in Figure 1). The diodes 9 supply the liquid crystal 4 with a sufficient threshold in relation to the voltage at the column electrodes 11 by means of voltages at the row electrodes 8 and provide the liquid crystal picture elements with a memory. In addition, liquid crystal orienting layers 10 are applied to the inner surfaces of the carrier plates 2 and 3. As is known, by applying a voltage to the liquid crystal layer 4, a changed orientation state of the liquid crystal molecules and thus an optically different state can be obtained. The display device can be implemented as both a transmitting and a reflecting device.

Figure 2 shows schematically a transmission/voltage characteristic of a display cell as occurs in the display device of Figure 1. Below a given threshold (V₁ or Vthr) the cell transmits almost no light, while above a given saturation voltage (V₂ or Vsat) it is completely transparent to light.

Figure 3 shows schematically a part of such a playback device. The picture elements 12 are connected via the picture electrodes 7 to column electrodes 11, which are arranged together with the row electrodes 8 in the form of a matrix in this embodiment. The picture elements 12 are connected to the row electrodes 8 via non-linear switching elements 9.

Figure 4 shows an equivalent circuit for a picture element 12, which is represented by the associated capacitance CLC and the capacitance of the associated non-linear switching element (in the high-impedance state) CNL, in order to calculate the crosstalk due to signal fluctuations at a column electrode 11. The non-linear element connected to a fixed voltage is considered to be connected to ground for the following description (based on the superposition principle). This non-linear element is not necessarily a ("back-to-back") diode, but may alternatively consist of diode rings, MIM switches, pip elements, nin elements, or other two-terminals, while CNL may also be a connection of the image electrode 6 via, for example, a multiplicity of diodes to various row electrodes, as is for example described in Dutch patent application 8502663 (equivalent to EP-A-0 217 466).

To control such a device, a control method is generally chosen in which

for the average voltage across a picture element (see Figure 2). In this method, the absolute value of the voltage across the picture elements 12 is essentially limited to the range between Vth and Vsat. This is described in more detail in "A LCTV Display Controlled by a-Si Diode Rings" by S. Togashi et al., SID '84, Digest pp. 324-5.

With this control around Vc, the point 13 when selected assumes an average voltage Vc = -½(Vsat+Vth) during the odd field duration and Vc = ½(Vsat+Vth) during the even field duration.

A good effect in terms of gradations (gray levels) is obtained if, depending on the information on the column electrode 11, the capacitor formed by the image electrode 12 is discharged or charged during control via the row electrodes 8 to voltage values between a maximum voltage Vc+Vdmax = Vsat and a minimum voltage Vc-Vdmax = Vth. Elimination of Vc results in Vdmax = ½(Vsat-Vth).

In the ideal case, the data voltage Vd at the column electrode 11 is:

-½(Vsat-Vth) ≤ Vd ≤ (½Vsat-Vth).

Since in practice this minimum or maximum voltage can be increased or decreased by crosstalk, the voltages Vd used in practice must be corrected so that the following applies to the corrected voltages Vx:

-Vx ≤ Vd ≤ Vx, with Vx > Vdmax .

The crosstalk for which compensation is necessary will now be calculated using Figures 3 and 4. If, for example, in a device for displaying images, a signal fluctuation Vx occurs at a column electrode 11, this leads to a signal change at point 13 (Figure 4), which belongs to a non-selected display element.

The maximum signal change at the column electrode 11 in the method according to the invention is at most Vx, because the data information is only present during a portion of the maximum time available for selection and because the reference voltage (0 volts) is then offered to the column electrode. The data voltage can of course also be 0 V first, and then the actual data voltage Vd can be offered during a portion of the maximum time available for selection.

Even if crosstalk compensation is used according to the method described in Dutch patent application 8601804, the maximum signal change at a column electrode is at most Vx, because (at a maximum signal Vx) the data voltage within the selection period first jumps from Vx to -Vx (a change of 2Vx) and then to 0 volts.

At point 13, where a signal Vx has just been written, such a voltage jump of Vx on line 11 can lead to a voltage of

lead.

For a satisfactory control of the liquid crystal element, Vx - ΔV must be exactly equal to Vdmax or

For the crosstalk term ΔVo this means:

If the data signal Vx is provided for a maximum time period Ts available for selection (64 µs in the PAL-SECAM system), the effective voltage Vp at the point 13 belonging to another picture element can be Vp = Vp + ΔVo due to crosstalk.

To prevent this crosstalk from affecting the image reproduction at a maximum of No grayscale (or color tones), the following must apply:

or

in other words, the maximum number of gray levels is

For a typical liquid crystal picture element (dimensions 300 µm x 300 µm, thickness approximately 8 µm, εr 6) and an a-Si-nin switch (dimensions approximately 20 µm x 20 µm, thickness of the i-layer approximately 400 nm), CLC 600 fF and CNL 120 fF apply, so that No ≤ 10. In the embodiment of the said patent application 8502663, approximately twice the value for CNL applies, since on both sides of the picture electrode a diode is arranged. For this, No ≤ 5 applies, which is too small for satisfactory reproduction.

If, as mentioned above, redundancy is desired, a single picture element can be divided into r sub-elements, each having its own driver element. This is shown schematically in Figures 5 and 6, in which the picture electrode 6 with the driver switching element 9 (Figure 5) is divided into three sub-electrodes 6a, 6b, 6c, each with its own driver element 9a, 9b, 9c (Figure 6). The picture electrode 7 corresponding to the picture electrode 6 is not divided.

When the image electrode is divided into sub-electrodes, the capacitance CLC also decreases. It can be roughly assumed that the number of gray levels initially decreases from N to N' = N/r as a result of crosstalk when the image element is divided into r sub-elements. In the examples given, when the image element is divided into three sub-electrodes, about 3 or about 1.5 levels remain available. Using redundancy is therefore pointless in this case.

However, when using a method according to the invention, the data information is offered during an m-th part of the maximum available selection period Ts, so that the effective voltage is now:

or

with mVp » ΔVo the following applies:

The following applies to the crosstalk signal ΔV₁:

N₁ gray levels can be achieved with this, provided that:

so that the maximum number of gray levels N₁ is now:

N₁ = 2mk = mNo.

By offering the data voltage during an m-th part of the available line selection time, the number of gray levels increases by approximately a factor of m.

A further increase is obtained if, after the data signal has been presented to the column electrode 11 of a selected cell during Ts/m, the inverse data signal is presented to the same column electrode 11 when the cell is no longer selected. The effective voltage Vp is then:

which can be transformed into:

so that with mVp²»2ΔVo²:

The latter can be transformed into

or

With Vth ≤ Vp ≤ Vsat the following applies:

so that for this control mode (with crosstalk compensation) the maximum number of grey levels N₂ applies:

For a liquid crystal (ZL1 84460, Merck) typically Vth = 2.1 volts, Vsat = 3.6 volts, so that for N₂ the following applies:

N2 = 1.4 m 4k²

or

N₂ = 1.4 m N2o.

It can be concluded that the number of gray levels associated with a conventional control (No) and the control according to the invention without (N₁) and with crosstalk compensation by signal inversion (N₂) is in this particular example:

No = 2k

N₁ = mNo

N₂ = 1.4 mN2o.

For k = 2.5 and 5, No = 5 and 10 respectively;

with m = 2 N₁ = 10 or 20,

(Ts/m = 32usec)

(PAL)

N₂ = 70 or 280,

with m = 8 N₁ = 40 or 80,

(Ts/m = 8usec)

N₂ = 280 and 1120 respectively.

With redundancy, in this last example, when split into three sub-electrodes (r = 3), the following applies:

N₁ 3 or 27,

N₂ 93 and 373 respectively.

The method according to the invention is therefore extremely suitable for the realization of gray levels in liquid crystal display devices.

Since the time period Ts/m is less than the maximum time period Ts available for selection, the switching element 9 is conductive during part of the line duration (which is 64 us for television applications, for example). The picture element is then not fully charged, but due to the steep characteristic curve of such elements, this is negligible. In addition, this voltage loss is almost the same for all switching elements, so that this can possibly be compensated for in the selection voltages. The selection voltages themselves can also be compensated for the forms of crosstalk described.

Figures 7 and 8 show the data information VD and the associated crosstalk signals ΔV₁, ΔV₂ for a device according to the invention with or without the crosstalk compensation described.

The compensation signal -VD can be obtained in a simple manner from the signal VD which is, for example, provided to a common input terminal 14 (see Figure 9) for a follower circuit 15 and for an inverter circuit 16, the outputs of which are connected to a column electrode 11 via switches 17, 18. By closing the switch 17 and then for a corresponding period of time the switch 18, the desired signal is obtained at the column electrode. The column electrode N then receives the reference signal because the switch 19 is closed while the switches 17, 18 remain open. The electrode 11 is now connected to the terminal 20 for the reference voltage via the switch 19. This case is shown in Figure 9. If no crosstalk compensation is used, the sub-branch 21 with the inverter circuit 16 and the switch 18 can be omitted. In this case, the follower circuit 15 can also be omitted if desired. The switch 19 is then complementary to switch 17, in other words, switch 17 is opened when switch 19 is closed and vice versa. If crosstalk compensation is used, switch 19 works complementarily to the circuit formed by the two branches 21, 22.

The scope of application of the invention is therefore not limited to the individual embodiments shown, but various modifications are possible.

For example, diode rings, MIM switches, nin, pip or pinip switches can be selected for the nonlinear switching elements, provided the switching speed is sufficiently high.

Various modifications are also possible when implementing the driver circuit in Figure 9.

In addition, different electro-optical media, such as electrophoretic suspensions or electrochromic substances, can be selected.

The embodiment is based on a switching method in which the data voltages on the picture elements switch around zero volts and the voltage swing on the picture elements remains limited to Vsat - Vth. The method according to the invention also offers the advantages mentioned with a different choice of data voltage and reference level. Possible deviations of the T-V curve from exponential behavior can be compensated in practice in a simple manner by suitable choice of data voltages, which are assigned to certain gray values.

Claims (12)

1. Method for controlling a display device (1) with an electro-optical display medium (4) between two carrier plates (2, 3), a system of picture elements (12) arranged in rows and columns, each picture element being formed by picture electrodes (6, 7) which are attached to mutually facing surfaces of the carrier plates, and a system of row electrodes (8) and column electrodes (11), a picture element row (12) being selected via the row electrodes (8) with the aid of non-linear switching elements (9) connected in series with the picture elements (12), and data signals being offered via the column electrodes (11), characterized in that a column electrode (11) is offered a data signal during part of the time period available for the selection of a picture element row (12), this data signal being offered almost simultaneously with a selection signal of the row electrode belonging to the picture element row (12), and in that a non-selection signal is presented to the row electrode (8) during the other part of the time period available for selection and in that in the absence of the data signal a reference voltage is applied to the column electrode (11). 2. Method according to claim 1 for use in a television display device, characterized in that the reference voltage is determined by the average value of the minimum data signal voltage in a first field and the maximum data signal voltage in a second field. 3. Method according to claim 1 or 2, characterized in that before the application of the reference voltage to the column electrode, the data signal changes its sign with respect to the reference voltage and the energy content of the partial signal thus obtained, which has a positive sign with respect to the reference voltage, is almost identical to that of the partial signal which has a negative sign with respect to the reference voltage, while one of the partial signals almost coincides with the selection signal. 4. Method according to one of claims 1 to 3, characterized in that the reference voltage is almost 0 volts. 5. Method according to claim 3 or 4, characterized in that the data signal consists of two partial signals of almost the same duration and these have almost identical absolute voltage values with regard to the reference voltage. 6. Method according to one of the preceding claims, characterized in that the duration of the data signal is between 2 us and 32 us. 7. Display device (1) for applying a method according to one of claims 1 to 6, with an electro-optical display medium (4) between two carrier plates (2, 3), a system of image elements (12) arranged in rows and columns, each image element being formed by image electrodes (6, 7) which are attached to mutually facing surfaces of the carrier plates, and a system of row electrodes (8) and column electrodes (11) for controlling the image electrodes (8) with the aid of non-linear switching elements (9), characterized in that a column electrode (11) is connected to a connection (14) for a signal to be displayed or to a connection (20) for a reference voltage via a parallel connection of two branches with complementary operating switches (17, 18, 19). 8. Playback device according to claim 7, characterized in that the branch connected to the terminal (14) for the signal to be played back contains a parallel circuit of two partial branches (21, 22) with switches, one of the partial branches (21), which contains an inverter circuit (16), being connected in series with the switch. 9. Display device according to claim 8, characterized in that the device also contains a driver circuit for the switches (17, 18, 19) which controls the switches in such a way that either the reference voltage or one of the partial signals received from the partial branches (21, 22) is offered to the column electrode (11). 10. Display device according to claim 9, characterized in that the partial signals offered at column electrodes (11) are almost equal in absolute value relative to the reference voltage and are each offered to the column electrode (11) for almost the same period of time. 11. Display device according to one of claims 7 to 10, characterized in that a picture electrode (6) is divided into a number of partial electrodes (6a, 6b, 6c) which are each operated via at least one non-linear switching element (9). 12. Display device according to one of claims 7 to 11, characterized in that the electro-optical medium is a liquid crystal, an electrophoretic suspension or an electrochromic substance.